Fix `Calling ->then in void context` warning (#1232)
Fix `Calling ->then in void context` warning (#1232)

PR #1205 added a call to `matrix_join_room`, but did not await the result,
which resulted in a bunch of warnings:

```
 ** Calling ->then in void context at tests/10apidoc/33room-members.pl line 82.
 ** IO::Async::Future=HASH(0x557d6befbf40) IO::Async::Future=HASH(0x557d6befbf40) lost a sequence Future at /home/rav/lib/perl5/Net/Async/HTTP/Connection.pm line 472.
```

Waiting for the join to complete before proceeding fixes the warnings.
matrix-org/sytest,matrix-org/sytest